While Computer-Tomographic(CT)-based Computer Assisted Surgery (CAS) systems are widely known in the art, CT-less CAS systems are slowly emerging as the technology of choice for North America and Europe. It is desirable to cut down the pre-operative time a surgeon must spend to prepare a surgery. It is also desirable to provide applications that can use other media than CT-scans, when these are not available. The CT-less system reduces pre-operative time and instrument calibration time, especially in simple surgeries, and in the case of more complex surgeries, the CT-less system can be combined with CT-based applications.
A CT-less intra-operative bone reconstruction system advantageously provides a surgeon with visual confirmation of the tasks he is performing during the surgery. In pending U.S. patent application Ser. No. 10/345,403 to the present applicant, there is described a method and system for intra-operatively presenting an approximate model of an anatomical structure by collecting a cloud of small surfaces. The cloud of small surfaces is gathered with a registration pointer having an adapted tip capable of making contact with the surface of an anatomical structure and registering the normal at the point of contact. Reconstructing and registering anatomical structures intra-operatively is at the core of CT-less CAS systems.
When performing surgery to the lower limbs, it is important to determine the mechanical axis of the leg. The mechanical axis refers to the axis formed by a line drawn from the center of the femoral head to the center of the knee joint and a line drawn from the center of the knee joint to the center of the ankle joint. In perfectly aligned leg, the mechanical axis forms a straight line.
Determining the mechanical axis of a leg comprises locating the center of the femoral head. It is known in the art of computer-assisted surgery to locate the center of the femoral head by dynamically registering the relative position of the femur while rotating the proximal end in a circular pattern. However, this technique is vulnerable to noise, thereby affecting the quality of the readings by the position sensing system. The level of accuracy obtained also varies depending on how long the rotation is maintained for and with how much precision the system can register the points while the bone is in motion. Furthermore, the motion of the femur for the registration process may cause the hipbone to move and this can introduce further errors into the measurements.
Detecting the femoral head is a crucial process that will influence the end result of the surgery. There is therefore a need to develop a system and method of femoral head detection that overcomes the drawbacks of the state of the art and guarantees a certain level of accuracy.